1. Field of the Invention
The present invention relates to copper foil optimal for chip-on-film (COF) mounting in fine pattern printed circuit boards.
2. Description of the Related Art
Along with the smaller size and lighter weight of electronic equipment, recent electronic devices have been integrated to advanced levels. Corresponding with this, the circuit patterns in the printed circuit boards have also been required to be made higher in density. Circuit patterns of fine line widths and interconnect pitches are now being formed. Particular advances are being made in increasing density in IC mounting boards for driving liquid crystal displays of PCs, mobile phones, and PDAs. The ICs are directly mounted on the board film, so this is called “chip-on-film” (COF) mounting.
With COF mounting, light passing through the film formed with the copper foil interconnect patterns is used to detect the IC positions. However, the visibility of conventional electrodeposited copper foil used for printed circuit boards (ability of detection of IC positions by light) is extremely poor. The reason for this is the high surface roughness of the copper foil. The parts of the film passing the light are parts where unnecessary copper foil other than the copper circuit parts has been etched away. The relief patterns of the surface of the copper foil are transferred to the film surface when bonding the copper foil to the film and remain there. When the light pass through the film, the amount of light which can proceed straight becomes smaller due to the relief patterns on the film surface and consequently the visibility becomes poorer.
Since the visibility is poor in conventional electrodeposited copper foil for printed circuit boards, two-layer materials comprised of polyimide film on which a copper layer is formed by sputtering (thin film formation process) and then copper is plated are used.
However, the bond strength of this sputtered copper layer with the film is weak and the etching solution or plating solution will erode between the copper foil and film at the time of forming the circuit resulting in the so-called “undercut” phenomenon. Further, since the bond strength is weak, the danger is harbored of the copper layer peeling off during use of the product.
On the other hand, as a method of treatment of copper foil for a circuit board, the method of roughening the surface of the copper foil by plating it by a copper-cobalt-nickel alloy comprised of copper and small amounts of cobalt and nickel has been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2-292894). The copper foil obtained by this roughening treatment is excellent in etchability, alkali etchability, and resistance to hydrochloric acid, but several defects have been pointed out such as the heat resistant peeling strength dropping when using an acrylic-based adhesive, the oxidation resistance also being not sufficient, and the color not becoming black, but brown to charcoal brown (for example, see Japanese Unexamined Patent Publication (Kokai) No. 9-87889). Practical application has therefore not yet been achieved.
Therefore, research has been conducted on improving the copper foil. Methods of forming three-layer structures calling for plating the surface of a copper foil with an alloy of copper, an amount of cobalt about 1/10 that of the copper, and an amount of nickel about 1/40 that of the copper, then forming a cobalt-nickel alloy plating layer over that, then forming a zinc plating layer (for example, see Japanese Unexamined Patent Publication (Kokai) No. 9-87888) or forming a zinc-nickel plating layer (for example, see Japanese Unexamined Patent Publication (Kokai) No. 9-87889) or methods of forming three-layer structures calling for roughening the surface of a copper foil by plating by an alloy of copper, an amount of cobalt about 1/10 that of the copper, and an amount of nickel about 1/40 that of the copper, then forming a cobalt plating layer, then further forming a zinc plating layer (for example, see Japanese Unexamined Patent Publication (Kokai) No. 8-335775) or forming a zinc-nickel plating layer (for example, see Japanese Unexamined Patent Publication (Kokai) No. 8-335776) have been disclosed. These methods have in particular been proposed as methods for treatment of copper foil reducing the drop in bond strength between a copper foil and a resin substrate occurring due to the much higher temperature of treatment performed in the process of production of circuit boards along with the smaller size and greater integration of semiconductor devices and the heat generated during use of apparatuses and thereby further improving the resistance to peeling under heat.
However, if employing the above three-layer structure improved copper foil for COF mounting, the following problem arises.
That is, the above copper foil is used to produce a circuit board satisfying the requirements for finer patterns and the ICs are mounted on the circuit board, but mounting the ICs on the circuit board requires that solder balls be placed on the copper foil circuits on the circuit board. To enable solder balls to be placed on the copper foil, the bondability with solder is raised by plating the copper foil with tin. The tin plating solution in this tin plating process dissolves the particles provided on the copper foil surface and ends up remarkably detracting from the peeling strength between the copper foil and circuit board.